are you able to login?..
If so, please do not startx the BT5, instead work in text mode and type: lspci | grep -i vga
Pleased post the output....
If not able to login, use liveDVD, open up a terminal and type as instructed above, please.
Need to know the chipset and see if supported..
You might want to use the open driver with microcode or ATI propietary driver.